A complex B chromosome system in the Korean field mouse, Apodemus peninsulae.

Information on B chromosomes of six subspecies of A. peninsulae Thomas, 1906, from 79 local populations of Russia (Siberia, Altai, Buryatia and the Far East), Mongolia, China, Korea and Japan (Hokkaido) is reviewed. The frequency of animals with B chromosomes is higher in this taxon than in other mammals and ranges from 0.4 up to 1.0, excluding two insular populations (Sakhalin Island and Stenin Island, Primorye) where Bs were not found. The B chromosome polymorphism shows four levels of variation in number (intraindividual mosaicism, intrapopulational and interpopulational), as well as variability in size, morphology and differential staining. Geographic variation was found among populations for these cytogenetic characteristics and, in some cases, it coincided with subspecies distribution. Comparative chromosome banding of micro and macro Bs illuminates possible pathways for their origin.